BOLD contrast fMRI of whole rodent tumour during air or carbogen breathing using echo-planar imaging at 1.5 T

The value of magnetic resonance blood oxygen level-dependent imaging in evaluating the efficacy of advanced cervical cancer combined with radiotherapy and chemotherapy

Background

Blood oxygen level-dependent magnetic resonance imaging (BOLD-MRI) is an imaging method used to analyze oxygenation status of the tumor.

Purpose

To investigate the feasibility of BOLD-MRI in evaluating the efficacy of advanced cervical cancer combined with radiotherapy and chemotherapy.

Material and Methods

This prospective study included 85 patients with advanced cervical cancer who received BOLD-MRI examination before and after concurrent chemoradiotherapy from October 2020 to December 2021. To investigate the changes of baseline R2* values and △R2* values of cervical cancers before and after treatment.

Results

29 cases were complete response, 34 cases were partial response, and 22 cases showed progression. The baseline R2* values of the tumors were lower than that of the normal cervical muscle ( P < 0.0001). After oxygen stimulation, the baseline R2* values of the tumors decreased ( P = 0.012). After treatment, the baseline R2* values of the tumors increased ( P = 0.007), and the dynamic △R2* values of the tumors decreased ( P = 0.025). The baseline R2* value of the complete response was the highest ( P = 0.000), the dynamic △R2* value of the complete response was the lowest ( P = 0.017).

Conclusion

BOLD-MRI can evaluate the efficacy of concurrent chemoradiotherapy for advanced cervical cancer.

Conjugated Photosensitizers for Imaging and PDT in Cancer Research

Early cancer detection and perfect understanding of the disease are imperative toward efficient treatments. It is straightforward that, for choosing a specific cancer treatment methodology, diagnostic agents undertake a critical role. Imaging is an extremely intriguing tool since it assumes a follow up to treatments to survey the accomplishment of the treatment and to recognize any conceivable repeating injuries. It also permits analysis of the disease, as well as to pursue treatment and monitor the possible changes that happen on the tumor. Likewise, it allows screening the adequacy of treatment and visualizing the state of the tumor. Additionally, when the treatment is finished, observing the patient is imperative to evaluate the treatment methodology and adjust the treatment if necessary. The goal of this review is to present an overview of conjugated photosensitizers for imaging and therapy.

Molecular imaging of tumor hypoxia with positron emission tomography

The problem of tumor hypoxia has been recognized and studied by the oncology community for over 60 years. From radiation and chemotherapy resistance to the increased risk of metastasis, low oxygen concentrations in tumors have caused patients with many types of tumors to respond poorly to conventional cancer therapies. It is clear that patients with high levels of tumor hypoxia have a poorer overall treatment response and that the magnitude of hypoxia is an important prognostic factor. As a result, the development of methods to measure tumor hypoxia using invasive and noninvasive techniques has become desirable to the clinical oncology community. A variety of imaging modalities have been established to visualize hypoxia in vivo. Positron emission tomography (PET) imaging, in particular, has played a key role for imaging tumor hypoxia because of the development of hypoxia-specific radiolabelled agents. Consequently, this technique is increasingly used in the clinic for a wide variety of cancer types. Following a broad overview of the complexity of tumor hypoxia and measurement techniques to date, this article will focus specifically on the accuracy and reproducibility of PET imaging to quantify tumor hypoxia. Despite numerous advances in the field of PET imaging for hypoxia, we continue to search for the ideal hypoxia tracer to both qualitatively and quantitatively define the tumor hypoxic volume in a clinical setting to optimize treatments and predict response in cancer patients.

Functional imaging of cerebral perfusion

The functional imaging of perfusion enables the study of its properties such as the vasoreactivity to circulating gases, the autoregulation and the neurovascular coupling. Downstream from arterial stenosis, this imaging can estimate the vascular reserve and the risk of ischemia in order to adapt the therapeutic strategy. This method reveals the hemodynamic disorders in patients suffering from Alzheimer's disease or with arteriovenous malformations revealed by epilepsy. Functional MRI of the vasoreactivity also helps to better interpret the functional MRI activation in practice and in clinical research.

In vivo MRI assessment of placental and foetal oxygenation changes in a rat model of growth restriction using blood oxygen level-dependent (BOLD) magnetic resonance imaging

OBJECTIVES

To evaluate whether changes in BOLD signal intensities following hyperoxygenation are related to intrauterine growth restriction (IUGR) in a rat model.

METHODS

IUGR was induced in pregnant rats by ligating the left vascular uterine pedicle at day 16 of gestation. BOLD MR imaging using a balanced steady-state free-precession (balanced-SSFP) sequence on a 1.5-T system was performed on day 19. Signal intensities (SI) before and after maternal hyperoxygenation were compared in the maternal liver and in control and growth-restricted foetoplacental units (FPUs).

RESULTS

Maternal hyperoxygenation resulted in a significant increase in SI in all regions of interest (P < 0.05) in the 18 rats. In the control group, the SI (mean ± SD) increased by 21 % ± 15 in placentas (n = 74) and 13 % ± 8.5 in foetuses (n = 53). In the IUGR group, the increase was significantly lower: 6.5 % ± 4 in placentas (n = 36) and 7 %± 5.5 in foetuses (n = 34) (P < 0.05).

CONCLUSION

BOLD MRI allows non-invasive assessment of the foetoplacental response to maternal hyperoxygenation in the rat and demonstrates its alteration in an IUGR model. This imaging method may provide a useful adjunct for the early diagnosis, evaluation, and management of human IUGR.

KEY POINTS

• Intra-uterine growth restriction is an important cause of perinatal morbidity and mortality. • Blood oxygen level-dependent MRI non-invasively assesses foetoplacental response to maternal hyperoxygenation. • In the rat, foetoplacental response to maternal hyperoxygenation is altered in IUGR. • Functional MRI may help to assess human IUGR.

Preclinical molecular imaging using PET and MRI

Molecular imaging fundamentally changes the way we look at cancer. Imaging paradigms are now shifting away from classical morphological measures towards the assessment of functional, metabolic, cellular, and molecular information in vivo. Interdisciplinary driven developments of imaging methodology and probe molecules utilizing animal models of human cancers have enhanced our ability to non-invasively characterize neoplastic tissue and follow anti-cancer treatments. Preclinical molecular imaging offers a whole palette of excellent methodology to choose from. We will focus on positron emission tomography (PET) and magnetic resonance imaging (MRI) techniques, since they provide excellent and complementary molecular imaging capabilities and bear high potential for clinical translation. Prerequisites and consequences of using animal models as surrogates of human cancers in preclinical molecular imaging are outlined. We present physical principles, values and limitations of PET and MRI as molecular imaging modalities and comment on their high potential to non-invasively assess information on hypoxia, angiogenesis, apoptosis, gene expression, metabolism, and cell trafficking in preclinical cancer research.

Analysing the response in R2* relaxation rate of intracranial tumours to hyperoxic and hypercapnic respiratory challenges: initial results

OBJECTIVE

To investigate the response in R2* relaxation rate of human intracranial tumours during hyperoxic and hypercapnic respiratory challenges.

METHODS

In seven patients with different intracranial tumours, cerebral R2* changes during carbogen and CO(2)/air inhalation were monitored at 3 T using a dynamic multigradient-echo sequence of high temporal and spatial resolution. The R2* time series of each voxel was tested for significant change. Regions of interest were analysed with respect to response amplitude and velocity.

RESULTS

The tumours showed heterogeneous R2* responses with large interindividual variability. In the 'contrast-enhancing' area of five patients and in the 'non-tumoral' tissue most voxels showed a decrease in R2* for carbogen. For the 'contrast-enhancing' area of two patients hardly any responses were found. In areas of 'necrosis' and perifocal 'oedema' typically voxels with R2* increase and no response were found for both gases. For tissue responding to CO(2)/air, the R2* changes were of the same order of magnitude as those for carbogen. The response kinetic was generally attenuated in tumoral tissue.

CONCLUSION

The spatially resolved determination of R2* changes reveals the individual heterogeneous response characteristic of intracranial human tumours during hyperoxic and hypercapnic respiratory challenges.

Intra- and inter-subject variability of high field fMRI digit maps in somatosensory area 3b of new world monkeys

This study evaluates the intra- and inter-subject variability of digit maps in area 3b of anesthetized squirrel monkeys. Maps were collected using high field blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI). BOLD responses to individual digit stimulations were mapped and their response properties (location, area of activation, % signal change, time to peak response) were compared within and across imaging sessions separated by up to 20 months. During single digit stimulation using a block design, the spatiotemporal response of the BOLD signal for individual runs within and across sessions and animals was well conserved, with a time to peak BOLD response of 20+/-4 s. The variability in the center of BOLD activation in area 3b was 0.41+/-0.24 mm (mean+/-SD) across individual 5-7 min runs within a scanning session and 0.55+/-0.15 mm across sessions. The average signal change across all animals, runs and sessions was 0.62+/-0.38%, and varied 32% within and 40% across sessions. In a comparison of the stability and reproducibility of the area of single digit activation obtained using three approaches, use of a fixed statistical threshold (P<10(-5)) yielded an average area of 4.8+/-3.5 mm(2) (mean+/-SD), adaptive statistical thresholding 1.32+/-1.259 mm(2) (mean+/-SD), and combined fixed statistical and adaptive BOLD signal amplitude 4.4+/-2.5 mm(2) (mean+/-SD) across image runs and sessions. The somatotopic organization was stable within animals across sessions, while across animals, there was some variation in overall activation pattern and inter-digit distances. These results confirm that BOLD activation maps of single digits in area 3b as characterized by activation center, signal amplitudes, and temporal profile are very stable. The activation sizes determined by various criteria are the most variable measure in this preparation, but adaptive statistical thresholding appears to yield the most stable and reproducible maps. This study serves as a baseline assessment of the limits imposed on the detection of plastic changes by experimental variations of the digit BOLD fMRI activation maps in normal animals, and as an indicator of the likely performance limits in human studies.

Positron Emission Tomography Imaging of Hypoxia

Hypoxia imaging has applications in functional recovery in ischemic events such as stroke and myocardial ischemia, but especially in tumors in which hypoxia can be predictive of treatment response and overall prognosis. Recently there has been development of imaging agents utilizing positron emission tomography for non-invasive imaging of hypoxia. Many of these PET agents have come to the forefront of hypoxia imaging. Halogenated PET nitroimidazole imaging agents labeled with (18)F (t(1/2) = 110 m) and (124)I (t(1/2) = 110 m) have been under investigation for the last 25 years, with radiometal agents ((64)Cu-ATSM) being developed more recently. This review focuses on these positron emission tomography imaging agents for hypoxia.

Imaging hypoxia in orthotopic rat liver tumors with iodine 124-labeled iodoazomycin galactopyranoside PET

PURPOSE

To evaluate iodine 124 (124I)-labeled iodoazomycin galactopyranoside (IAZGP) positron emission tomography (PET) in the detection of hypoxia in an orthotopic rat liver tumor model by comparing regions of high (124)I-IAZGP uptake with independent measures of hypoxia and to determine the optimal time after injection to depict hypoxia.

MATERIALS AND METHODS

The institutional animal care and use committee approved this study. Morris hepatoma tumors were established in the livers of 15 rats. Tumor oxygenation was measured in two rats with a fluorescence fiberoptic oxygen probe. (124)I-IAZGP was coadministered with the established hypoxia markers pimonidazole and EF5 in nine rats; 12-hour PET data acquisition was performed 24 hours later. Tumor cryosections were analyzed with immunofluorescence and autoradiography. In the four remaining rats, serial 20- and 60-minute PET data acquisition was peformed up to 48 hours after tracer administration.

RESULTS

Oxygen probe measurements showed severe hypoxia (<1 mm Hg) distributed evenly throughout tumor tissue. Analysis of cryosections showed diffuse homogeneous uptake of (124)I-IAZGP throughout all tumors. The (124)I-IAZGP distribution correlated positively with pimonidazole (r = 0.78) and EF5 (r = 0.76) distribution. Tracer uptake in tumors was detectable with PET after 24 hours in seven of nine rats. In rats that underwent serial PET, tumor-to-liver contrast was sufficient to enable detection of hypoxia between 6 and 48 hours after tracer administration. The optimal ratio between signal intensity and tumor-to-liver contrast occurred 6 hours after tracer administration.

CONCLUSION

Regions of high (124)I-IAZGP uptake in orthotopic rat liver tumors are consistent with independent measures of hypoxia; visualization of hypoxia with (124)I-IAZGP PET is optimal 6 hours after injection.

Intermittent hypoxia is a key regulator of cancer cell and endothelial cell interplay in tumours

Solid tumours are complex structures in which the interdependent relationship between tumour and endothelial cells modulates tumour development and metastasis dissemination. The tumour microenvironment plays an important role in this cell interplay, and changes in its features have a major impact on tumour growth as well as on anticancer therapy responsiveness. Different studies have shown irregular blood flow in tumours, which is responsible for hypoxia and reoxygenation phases, also called intermittent hypoxia. Intermittent hypoxia induces transient changes, the impact of which has been underestimated for a long time. Recent in vitro and in vivo studies have shown that intermittent hypoxia could positively modulate tumour development, inducing tumour growth, angiogenic processes, chemoresistance, and radioresistance. In this article, we review the effects of intermittent hypoxia on tumour and endothelial cells as well as its impacts on tumour development.

Diffusion weighted imaging in small rodents using clinical MRI scanners

Diffusion weighted imaging (DWI) has emerged as a unique and powerful non-invasive magnetic resonance imaging (MRI) technique with a major potential impact on imaging-based diagnosis in a variety of clinical applications including oncology and tissue viability assessment. In light of increasing demand for applying this technique in preclinical investigations using small animals, we have explored the potentials of a clinical magnet for acquiring the DWI in rats and mice with either cerebral ischemia or solid tumors. Through technical adaptation and optimization, we have been able to perform a series of clinically relevant animal studies with conclusions based on DWI quantification. Focusing more on practical aspects and cross-referencing with the current literature, this paper is aimed to summarize our ongoing DWI studies on small rodents with stroke and tumors, and to provide protocols for researchers to replicate similar techniques in their own preclinical and clinical studies.

Current issues and perspectives in small rodent magnetic resonance imaging using clinical MRI scanners

Small rodents such as mice and rats are frequently used in animal experiments for several reasons. In the past, animal experiments were frequently associated with invasive methods and groups of animals had to be killed to perform longitudinal studies. Today's modern imaging techniques such as magnetic resonance imaging (MRI) allow non-invasive longitudinal monitoring of multiple parameters. Although only a few institutions have access to dedicated small animal MR scanners, most institutions carrying out animal experiments have access to clinical MR scanners. Technological advances and the increasing field strength of clinical scanners make MRI a broadly available and viable technique in preclinical in vivo research. This review provides an overview of current concepts, limitations, and recent studies dealing with small animal imaging using clinical MR scanners.

Dynamics of tumor hypoxia measured with bioreductive hypoxic cell markers

Hypoxic cells are common in tumors and contribute to malignant progression, distant metastasis and resistance to radiotherapy. It is well known that tumors are heterogeneous with respect to the levels and duration of hypoxia. Several strategies, including high-oxygen-content gas breathing, radiosensitizers and hypoxic cytotoxins, have been developed to overcome hypoxia-mediated radioresistance. However, with these strategies, an increased tumor control rate is often accompanied by more severe side effects. Consequently, development of assays for prediction of tumor response and early monitoring of treatment responses could reduce both over- and undertreatment, thereby avoiding unnecessary side effects. The purpose of this review is to discuss different assays for measurement of hypoxia that can be used to detect changes in oxygen tension. The main focus is on exogenous bioreductive hypoxia markers (2-nitroimidazoles) such as pimonidazole, CCI-103F, EF5 and F-misonidazole. These are specifically reduced and bind to macromolecules in viable hypoxic cells. A number of these bioreductive drugs are approved for clinical use and can be detected with methods ranging from noninvasive PET imaging (low resolution) to microscopic imaging of tumor sections (high resolution). If the latter are stained for multiple markers, hypoxia can be analyzed in relation to different microenvironmental parameters such as vasculature, proliferation and endogenous hypoxia-related markers, for instance HIF1alpha and CA-IX. In addition, temporal and spatial changes in hypoxia can be analyzed by consecutive injection of two different hypoxia markers. Therefore, bioreductive exogenous hypoxia markers are promising as tools for development of predictive assays or as tools for early treatment monitoring and validation of potential endogenous hypoxia markers.

Evaluation of hypoxia in an experimental rat tumour model by [(18)F]fluoromisonidazole PET and immunohistochemistry

This study aimed to evaluate tumour hypoxia by comparing [¹⁸F]Fluoromisonidazole uptake measured using positron emission tomography ([¹⁸F]FMISO-PET) with immunohistochemical (IHC) staining techniques. Syngeneic rhabdomyosarcoma (R1) tumour pieces were transplanted subcutaneously in the flanks of WAG/Rij rats. Tumours were analysed at volumes between 0.9 and 7.3 cm³. Hypoxic volumes were defined using a 3D region of interest on 2 h postinjection [¹⁸F]FMISO-PET images, applying different thresholds (1.2–3.0). Monoclonal antibodies to pimonidazole (PIMO) and carbonic anhydrase IX (CA IX), exogenous and endogenous markers of hypoxia, respectively, were used for IHC staining. Marker-positive fractions were microscopically measured for each tumour, and hypoxic volumes were calculated. A heterogeneous distribution of hypoxia was observed both with histology and [¹⁸F]FMISO autoradiography. A statistically significant correlation (P<0.05) was obtained between the hypoxic volumes defined with [¹⁸F]FMISO-PET and the volumes derived from the PIMO-stained tumour sections (r=0.9066; P=0.0001), regardless of the selected threshold between 1.4 and 2.2. A similar observation was made with the CA IX staining (r=0.8636; P=0.0006). The relationship found between [¹⁸F]FMISO-PET and PIMO- and additionally CA IX-derived hypoxic volumes in rat rhabdomyosarcomas indicates the value of the noninvasive imaging method to measure hypoxia in whole tumours.

Comparison of BOLD contrast and Gd-DTPA dynamic contrast-enhanced imaging in rat prostate tumor

The microcirculation and oxygenation of a tumor play important roles in its responsiveness to cytotoxic treatment, and noninvasive assessments of its vascular properties may have prognostic value. Dynamic contrast-enhanced (DCE) (1)H MRI based on infusion of Gd-DTPA, and blood oxygen level-dependent (BOLD) contrast based on altering inhaled gas are both sensitive to vascular characteristics. This study compares the effects observed in eight Dunning prostate R3327-AT1 rat tumors imaged sequentially at 4.7 Tesla by echo-planar imaging (EPI). Both interventions generated a significant response, and each revealed significant differences between the center and periphery of the tumors. On a voxel-by-voxel basis across the whole tumor population, there was a close correlation between the maximum rate of signal response and the magnitude of response to each intervention (R(2) >or= 0.6, P < 0.0001). However, when the data were analyzed separately for each individual tumor, some showed a weak correlation (R(2) < 0.4), particularly for DCE, and the nature (slope) varied between separate tumors. Generally, there was a weak correlation (N = 7, R(2) < 0.5) between responses to the two interventions on a tumor-by-tumor basis, which emphasizes that the techniques are not equivalent. Both techniques revealed intra- and intertumor heterogeneity, but the BOLD response was more rapidly reversible than the DCE response. This suggests that the BOLD technique may be a useful tool for investigating interventions (such as drugs) that cause vascular disruption.

Cluster analysis of BOLD fMRI time series in tumors to study the heterogeneity of hemodynamic response to treatment

BOLD-contrast functional MRI (fMRI) has been used to assess the evolution of tumor oxygenation and blood flow after treatment. The aim of this study was to evaluate K-means-based cluster analysis as a exploratory, data-driven method. The advantage of this approach is that it can be used to extract information without the need for prior knowledge concerning the hemodynamic response function. Two data sets were acquired to illustrate different types of BOLD fMRI response inside tumors: the first set following a respiratory challenge with carbogen, and the second after pharmacological modulation of tumor blood flow using flunarizine. To improve the efficiency of the clustering, a power density spectrum analysis was first used to isolate voxels for which signal changes did not originate from noise or linear drift. The technique presented here can be used to assess hemodynamic response to treatment, and especially to display areas of the tumor with heterogeneous responses.

How does blood oxygen level-dependent (BOLD) contrast correlate with oxygen partial pressure (pO2) inside tumors?

Blood oxygen level-dependent (BOLD) contrast-based functional MRI (fMRI) has been reported as a method to assess the evolution of tumor oxygenation after hyperoxic treatments, because of its sensitivity to changes in blood flow and deoxyhemoglobin content. However a number of questions remain: 1) In view of tumor heterogeneity, how good is the correlation between the MR parameters in gradient-echo imaging (signal intensity (SI) or effective transverse relaxation time (T(*)(2))) and local tumor oxygen partial pressure (pO(2))? 2) Is the magnitude of the change in SI or T(*)(2) a quantitative marker for variation in pO(2)? 3) Is initial T(*)(2) a good marker for initial pO(2)? To address these questions, murine tumors were imaged during respiratory challenges at 4.7 Tesla, using fiber-optic microprobes to simultaneously acquire tumor pO(2) and erythrocyte flux. The BOLD signal response (SI and T(*)(2)) was temporally correlated with changes in pO(2). However, the magnitude of the signal bore no absolute relation to pO(2) across tumors, i.e., a given change in SI corresponded to a 25 mmHg pO(2) change in one tumor, but to a 100 mmHg change in another. The initial T(*)(2) value did not reliably predict tumor oxygenation at the beginning of the experiment. In conclusion, the major advantages of the technique include noninvasiveness, high spatial resolution, and real-time detection of pO(2) fluctuations. Information afforded by the BOLD imaging technique is qualitative in nature and may be combined with other techniques capable of providing an absolute measure of pO(2).

In vivo animal functional MRI: improved image quality with a body-adapted mold

PURPOSE

To reduce functional magnetic resonance imaging (fMRI) susceptibility distortion at the air/tissue interphase in animal experiments.

MATERIALS AND METHODS

We investigated the applicability of a body-adaptable flexible mold consisting of a fast-setting alginate. This technique was implemented for subcutaneous growing tumors in rats and for the brains of monkeys.

RESULTS

The T(2)*-weighted gradient-echo, echo-planar imaging (GE-EPI) data obtained with the body-adapted mold showed a reduction of susceptibility artifacts and improved image quality. With both rat tumor and monkey brain, an optimized match with the anatomical T(1) images was possible.

CONCLUSION

The present mold methodology is a rapid, easy, and inexpensive way to reduce magnetic susceptibility during animal GE-EPI.